docs: def buildprior(self, prior, mopt=None, extend=False):
          " Extract the model's parameters from prior. "
          newprior = {}
          # allow for log-normal, etc priors
          intercept, slope = gv.get_dictkeys(
              prior, [self.intercept, self.slope]
              )
          newprior[intercept] = prior[intercept]
          if mopt is None:
              # slope parameter marginalized if mopt is not None
              newprior[slope] = prior[slope]
          return newprior